%%%-------------------------------------------------------------------
%%% @doc UDP heartbeat sender for iot host liveness.
%%% @end
%%%-------------------------------------------------------------------
-module(efka_iot_heartbeat).

-behaviour(gen_server).

%% API
-export([start_link/0]).

%% gen_server callbacks
-export([init/1, handle_call/3, handle_cast/2, handle_info/2, terminate/2, code_change/3]).

-define(SERVER, ?MODULE).
-define(HEARTBEAT_VERSION, 1).
-define(HEARTBEAT_NONCE_BYTES, 16).
-define(DEFAULT_INTERVAL, 5000).

-record(state, {
    socket :: gen_udp:socket(),
    host :: inet:hostname() | inet:ip_address(),
    port :: inet:port_number(),
    interval :: pos_integer(),
    uuid :: binary(),
    heartbeat_secret :: binary()
}).

%%%===================================================================
%%% API
%%%===================================================================

-spec start_link() -> {ok, pid()} | ignore | {error, term()}.
start_link() ->
    gen_server:start_link({local, ?SERVER}, ?MODULE, [], []).

%%%===================================================================
%%% gen_server callbacks
%%%===================================================================

-spec init(list()) -> {ok, #state{}} | {stop, term()}.
init([]) ->
    ok = application:ensure_started(crypto),
    {ok, ServerProps} = application:get_env(efka, iot_server),
    {ok, AuthProps} = application:get_env(efka, auth),
    HeartbeatProps = case application:get_env(efka, heartbeat) of
        {ok, Props} -> Props;
        undefined -> []
    end,
    Host = proplists:get_value(host, ServerProps),
    UdpPort = proplists:get_value(udp_port, ServerProps),
    Interval = proplists:get_value(interval, HeartbeatProps, ?DEFAULT_INTERVAL),
    UUID = list_to_binary(proplists:get_value(uuid, AuthProps)),
    Token = list_to_binary(proplists:get_value(token, AuthProps)),
    HeartbeatSecret = crypto:hash(sha256, Token),
    case gen_udp:open(0, [binary]) of
        {ok, Socket} ->
            erlang:send_after(0, self(), heartbeat),
            {ok, #state{
                socket = Socket,
                host = Host,
                port = UdpPort,
                interval = Interval,
                uuid = UUID,
                heartbeat_secret = HeartbeatSecret
            }};
        {error, Reason} ->
            {stop, Reason}
    end.

-spec handle_call(term(), {pid(), term()}, #state{}) -> {reply, ok, #state{}}.
handle_call(_Request, _From, State) ->
    {reply, ok, State}.

-spec handle_cast(term(), #state{}) -> {noreply, #state{}}.
handle_cast(_Request, State) ->
    {noreply, State}.

-spec handle_info(term(), #state{}) -> {noreply, #state{}}.
handle_info(heartbeat, State = #state{interval = Interval}) ->
    ok = send_heartbeat(State),
    erlang:send_after(Interval, self(), heartbeat),
    {noreply, State};
handle_info(Info, State) ->
    logger:warning("[efka_iot_heartbeat] ignore unknown info: ~p", [Info]),
    {noreply, State}.

-spec terminate(term(), #state{}) -> ok.
terminate(_Reason, #state{socket = Socket}) ->
    gen_udp:close(Socket),
    ok.

-spec code_change(term(), #state{}, term()) -> {ok, #state{}}.
code_change(_OldVsn, State, _Extra) ->
    {ok, State}.

%%%===================================================================
%%% Internal functions
%%%===================================================================

-spec send_heartbeat(#state{}) -> ok.
send_heartbeat(#state{socket = Socket, host = Host, port = Port, uuid = UUID, heartbeat_secret = HeartbeatSecret}) ->
    Packet = heartbeat_packet_iodata(UUID, HeartbeatSecret),
    case gen_udp:send(Socket, Host, Port, Packet) of
        ok ->
            ok;
        {error, Reason} ->
            logger:warning("[efka_iot_heartbeat] send heartbeat failed, reason: ~p", [Reason]),
            ok
    end.

-spec heartbeat_packet_iodata(binary(), binary()) -> iodata().
heartbeat_packet_iodata(UUID, HeartbeatSecret) when is_binary(UUID), is_binary(HeartbeatSecret) ->
    UUIDLen = byte_size(UUID),
    Timestamp = efka_util:timestamp(),
    Nonce = crypto:strong_rand_bytes(?HEARTBEAT_NONCE_BYTES),
    Payload = <<?HEARTBEAT_VERSION:8, UUIDLen:16, UUID:UUIDLen/binary, Timestamp:64/unsigned-big, Nonce/binary>>,
    Mac = crypto:mac(hmac, sha256, HeartbeatSecret, Payload),
    [Payload, Mac].